Traffic flow at a T-junction: A sufficient condition for a left-turn lane

A previous model of a T-junction on a narrow, two-lane, major road yields a formula for the proportion of traffic configurations in which cars travelling past the junction along the major road are brought to a halt by cars turning left into the minor road. The proportion is expressed in terms of four parameters, namely, the two mean traffic flows on the major road, the probability of a left turn into the minor road, and the minimum time gap in the oncoming flow which that maneuver requires. If this proportion is higher than is deemed acceptable, then a left-turn lane is needed. The proportion was calculated in a previous article, but recently found errors invalidate that calculation (though not the model). Here the errors are corrected to yield a new sufficient condition for a left-turn lane at a T-junction.